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Citrus Diseases Exotic to Florida: Mal Secco1 K.-R

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Citrus Diseases Exotic to Florida: Mal Secco1 K.-R Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. PP290 Citrus Diseases Exotic to Florida: Mal Secco1 K.-R. Chung2 Introduction Citrus is susceptible to a large number of diseases caused by plant pathogens. Plant diseases that affect citrus can cause severe economic losses, but, fortunately, not all pathogens attacking citrus worldwide are present in Florida. Any ex- otic disease, if introduced, has the potential to significantly increase production costs and thus decrease profitability for Florida growers. This series of EDIS fact sheets (available at http://edis.ifas.ufl.edu/topic_series_citrus_diseases_ex- otic_to_florida) presents background information for each exotic citrus disease in order to provide a basis for evaluating exotic pathogens that may pose potential risks Figure 1. A sour orange leaf affected by mal secco disease, showing to Florida citrus and to facilitate a decision-making frame- vein chlorosis. Credits: Reprinted with permission from Figure 2 of Q. work for preventing their introduction and spread. This Migheli, et al., “Mal Secco Disease Caused by Phoma tracheiphila: A paper discusses mal secco (Italian for dry disease), which Potential Threat to Lemon Production Worldwide,” Plant Disease (2009) 93: 852-67. Courtesy of American Phytopathological Society (APS) is caused by the fungus Phoma tracheiphila (synonym: Press. Deuterophoma tracheiphila). Concerns about Mal Secco Causal Agent of Mal Secco The causal agent of mal secco is the fungus Phoma tra- Mal secco is a fungal disease that causes serious damage cheiphila (syn. Deuterophoma tracheiphila). P. tracheiphila to a number of citrus cultivars in the Mediterranean and can be easily isolated and cultured from infected citrus Black Sea areas (with the exceptions of Spain, Portugal, and trees. Isolates vary in pathogenicity and pigment produc- Morocco). The fungus spreads systemically in the tree after tion. Some less virulent types of P. tracheiphila have also initial infection and then kills infected limbs. It often causes been reported. a complete collapse of the whole tree if infection occurs on the trunk or low in the canopy. The disease has been known for a century in the Mediterranean area, but has not been Cultivars Affected by Mal Secco reported in other citrus-growing regions. Under favorable conditions, mal secco can affect a range of citrus cultivars and relatives, including Fortunella, Poncirus, and Severina. However, mal secco is most severe 1. This document is PP290, one of a series of the Plant Pathology Department, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date February 2012. Visit the EDIS website at http://edis.ifas.ufl.edu. 2. K.-R. Chung, associate professor, Plant Pathology Department, Citrus Research and Education Center, Lake Alfred, FL; Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville, FL 32611. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A&M University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer-Chancy, Interim Dean Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. on lemon, citron (Citrus medica L.), lime (C. latifolia Tan.), and bergamot (C. bergamia Risso). Under greenhouse conditions, P. tracheiphila is able to infect almost all citrus species that are wounded before inoculation. In fields, the fungus rarely affects oranges and grapefruit. However, P. tracheiphila infections of some tangelos, tangors, and mandarins have been reported. While infections most commonly occur in the scion, infection can also occur in rootstock sprouts. Rough lemon, alemow, sour oranges, volkamer lemon, and citranges are all susceptible. Trees of any age can become infected, but the disease is especially severe in young plantings. Typical Symptoms of Mal Secco Symptoms include vein chlorosis (Figure 1) and wilting of leaves, followed by twig and/or limb dieback. The vascular system in an affected tree is often clogged by gum and fungal mycelium. The wood of infected limbs often shows red or yellow streaks, which are diagnostic for the disease (Figures 2 and 3). Infections in the canopy move downward Figure 3. Orange-reddish discoloration of an orange limb affected by slowly, while infections at the base of the tree may move mal secco disease. Credits: Reprinted with permission from Figure 6 of upward rapidly. Symptoms apparently are associated with Q. Migheli, et al., “Mal Secco Disease Caused by Phoma tracheiphila: A Potential Threat to Lemon Production Worldwide,” Plant Disease (2009) toxins and cell-wall degrading enzymes released by the 93: 852-67. Courtesy of APS Press. causal fungus. The affected trees quickly decline and die. Transmission of Mal Secco The fungus forms a spherical or inversely pear-shaped pycnidium (an asexual fruit body) on twigs, leaves, and fruit. Local spread of P. tracheiphila is by rain-dispersed pycnidiospores (asexual spores) produced from pycnidia (singular pycnidium). Spores can penetrate leaves directly, but infections typically occur through wounds that may be caused by hail, frost, harvesting injury, or pruning. Infec- tions can also occur through exposed roots. Most infections occur during cool periods with rainy weather. Spread within the xylem of the infected trees is associated with the movement of spores. Long distance spread usually occurs through the movement of infected nursery stock. Detecting Mal Secco in the Field The main diagnostic field symptom is that wood of affected limbs has a reddish-yellow stain (Figures 2 and 3). This symptom can be seen by making a tangential cut into an Figure 2. Orange-reddish discoloration of the ‘Femminello’ lemon affected limb. Isolation and pathogenicity tests can confirm affected by mal secco. Credits: Dr. Santa Olga Cacciola and Dr. Gaetano the identity of the causal agent. Serological and molecular Magnano di San Lio. assays have also been developed to test for mal secco. Control of Mal Secco Long distance spread of mal secco can be prevented by using disease-free nursery materials. Pruning dry twigs 2 Archival copy: for current recommendations see http://edis.ifas.ufl.edu or your local extension office. and limbs can decrease the rate of spread into the trunk, within affected trees, and to adjacent trees. Removal of suckers and root sprouts reduces the chance of infection in the trunk and main scaffolds. Wounding should be avoided during rainy periods. In Italy, copper-based fungicides are effective for controlling mal secco if applied every 2–4 weeks between October and February. Lemons that are tolerant of mal secco have been identified, but they lack the market appeal of susceptible cultivars. Pre-infection with either citrus exocortis viroid or a hypovirulent isolate of P. tracheiphila has been shown to reduce development of mal secco symptoms. What Can Growers Do? While mal secco is a serious problem for certain citrus areas in the Mediterranean, its failure to spread to other regions suggests that climatic conditions limit its distribution or that it is not easily spread to new areas. Better information on the potential susceptibility of Florida cultivars, especially newer releases, would be useful to assess the future threat. Preventing mal secco from entering Florida is much easier than trying to eradicate or control it. Any citrus-propa- gating materials must be introduced through the Florida Department of Agricultural and Consumer Services, Division of Plant Industry to ensure healthy plants and the economic viability of the Florida citrus industry. 3.
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